Eighty Shades of Option Key
Guillaume Gète (via Glenn Fleishman):
The little guide lists seventy various tips, from the particularly useful to the very futile... Obviously, it is not a question here of being exhaustive, but rather to give ways to encourage you remember to press the Option key during all your manipulations with the mouse and the keyboard... in order to save you time and make you more productive, even... have fun!
See also: TidBITS-Talk.
Update (2023-05-16): gsymon:
There’s also Apple Help/Support, which is fairly comprehensive. I think I may even have printed it out many years back.
Update (2023-05-17): Sarah Reichelt:
And if you’re just starting to use keyboard shortcuts, or find this list overwhelming, check out an article I wrote for the people at Timing app (my favourite time tracker) about using keyboard shortcuts on your Mac.